When boiler tubes are fouled with boiler sludge (mainly iron oxides) the water in the porous deposit layer is stagnant. Locally wick boiling will occur, often resulting in deposition of silicates, sulphates. Because of the growing solid silicate or sulphate layer the metal temperature will increase locally. Acid boiler water, pa from chloride acid forming seawater salts because of a seawater condenser inleakage, will concentrate in cracks or pores in the solid deposit layer and result in high concentrating ferrous-chloride solutions.
In total 62 different steel heats were tested in ferrous chloride solutions at 310 °C (10 MPa) in autoclaves during 4 days. After the exposure the morphology of the corrosion scale was studied.
It appeared that at low ferrous chloride concentrations a still well protective adhering magnetite layer was formed. In the high concentrated solutions a laminated non-protective oxide layer was formed. Test solutions of 13 various ferrous chloride concentrations were chosen ranging from 0.005 up to 0.375 molar ferrous chloride.
Amazing is that each of the 62 tested carbon steel heat had a critical concentration above which the oxide layer became non-protective.
Critical ferrouschloride concentration above which the weight loss increases sharply, corresponding with the point where laminated oxide structure is formed.
This critical concentration was considered as the corrosion sensitivity of the steel. The steel samples originated from waterwall tubes of boilers in Dutch electrical power stations. The tubes were cut from the evaporators because of corrosion failures or because of inspection for fouling of the boiler.
Results of critical chloride
By correlating the critical ferrous chloride concentration with the steel composition a regression equation was found with which the critical ferrous chloride concentration for new steels can be calculated.
Critical ferrous chloride = 0.1 Mn + 2 P +0.2 Cr + 0.04 Mo – 0.05
From experience with corrosion in boilers it became clear that in case of severe corrosion in boilers the critical ferrous chloride value was always less than 0.035 molar.
• Aside from water conditioning and heat flux conditions specification for the steel regarding corrosion resistance can be used.
• In case of failures or new boilers it is advised to look after the chemical composition of the steel and to bring that fact in discussion at the boiler manufacturer.
• Boiler steels should have a chemical composition that they can resist a 0.035 molar FeCl2 solution.
See for more information papers nr: 9, 11, 13, 14, 17, 20, 38, 46 and 61.